Pulse oximeter and sensor optimized for low saturation

ABSTRACT

Embodiments of the present invention relate to a pulse oximeter sensor. Specifically, embodiments of the present invention include emitting infrared light from a first light source into tissue, the infrared light including an infrared wavelength spectrum useful for measuring oxygen saturation in a patient with high saturation, emitting far red light from a second light source into the tissue, the far red light including a wavelength between 700 and 790 nanometers, and detecting the infrared light and far red light from the first and second light sources with a detector after the light has been scattered by the tissue.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/698,962, filed Oct. 30, 2003, which is a continuation of U.S.application Ser. No. 09/882,371, filed Jun. 14, 2001, now U.S. Pat. No.6,662,033, which is a continuation of U.S. application Ser. No.09/003,413, filed Jan. 6, 1998, now U.S. Pat. No. 6,272,363, which is acontinuation of U.S. application Ser. No. 08/413,578, filed Mar. 30,1995, now U.S. Pat. No. 5,782,237, which is a continuation-in-part ofU.S. application Ser. No. 08/221,911, filed Apr. 1, 1994, now U.S. Pat.No. 5,421,329, the disclosures of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

Pulse oximetry is used to continuously monitor the arterial blood oxygensaturation of adults, pediatrics and neonates in the operating room,recovery room, intensive care units, and increasingly on the generalfloor. A need exists for pulse oximetry in the delivery room formonitoring the oxygen status of a fetus during labor and delivery, andfor monitoring the oxygen status of cardiac patients.

Pulse oximetry has traditionally been used on patient populations wherearterial blood oxygen saturation is typically greater than 90%, i.e.,more than 90% of the functional hemoglobin in the arterial blood isoxyhemoglobin and less than 10% is reduced hemoglobin. Oxygen saturationin this patient population rarely drops below 70%. When it does drop tosuch a low value, an unhealthy clinical condition is indicated, andintervention is generally called for. In this situation, a high degreeof accuracy in the estimate of saturation is not clinically relevant, asmuch as is the trend over time.

Conventional two wavelength pulse oximeters emit light from two LightEmitting Diodes (LEDs) into a pulsatile tissue bed and collect thetransmitted light with a photodiode positioned on an opposite surface(transmission pulse oximetry), or an adjacent surface (reflectance pulseoximetry). The LEDs and photodetector are housed in a reusable ordisposable sensor which connects to the pulse oximeter electronics anddisplay unit. The “pulse” in pulse oximetry comes from the time varyingamount of arterial blood in the tissue during the cardiac cycle, and theprocessed signals from the photodetector create the familiarplethysmographic waveform due to the cycling light attenuation. Forestimating oxygen saturation, at least one of the two LEDs' primarywavelength must be chosen at some point in the electromagnetic spectrumwhere the absorption of oxyhemoglobin (HbO₂) differs from the absorptionof reduced hemoglobin (Hb). The second of the two LEDs' wavelength mustbe at a different point in the spectrum where, additionally, theabsorption differences between Hb and HbO₂ are different from those atthe first wavelength. Commercial pulse oximeters utilize one wavelengthin the near red part of the visible spectrum near 660 nanometers (nm),and one in the near infrared part of the spectrum in the range of880-940 nm (See FIG. 1). As used herein, “red” wavelengths or “red”spectrum will refer to the 600-800 nm portion of the electromagneticspectrum; “near red”, the 600-700 nm portion; “far red”, the 700-800 nmportion; and “infrared” or “near infrared”, the 800-1000 nm portion.

Photocurrents generated within the photodetector are detected andprocessed for measuring the modulation ratio of the red to infraredsignals. This modulation ratio has been observed to correlate well toarterial oxygen saturation as shown in FIG. 2. Pulse oximeters and pulseoximetry sensors are empirically calibrated by measuring the modulationratio over a range of in vivo measured arterial oxygen saturations(SaO₂) on a set of patients, healthy volunteers or animals. The observedcorrelation is used in an inverse manner to estimate saturation (SpO₂)based on the real-time measured value of modulation ratios. (As usedherein, SaO₂ refers to the in vivo measured functional saturation, whileSpO₂ is the estimated functional saturation using pulse oximetry.)

The choice of emitter wavelengths used in conventional pulse oximetersis based on several factors including, but not limited to, optimumsignal transmission through blood perfused tissues, sensitivity tochanges in arterial blood oxygen saturation, and the intensity andavailability of commercial LEDs at the desired wavelengths.Traditionally, one of the two wavelengths is chosen from a region of theabsorption spectra (FIG. 1) where the extinction coefficient of HbO₂ ismarkedly different from Hb. The region near 660 nm is where the ratio oflight absorption due to reduced hemoglobin to that of oxygenatedhemoglobin is greatest. High intensity LEDs in the 660 nm region arealso readily available. The IR wavelength is typically chosen near 805nm (the isosbestic point) for numerical convenience, or in the 880-940nm spectrum where additional sensitivity can be obtained because of theinverse absorption relationship of Hb and HbO₂. Unfortunately, pulseoximeters which use LED wavelengths paired from the 660 nm band and 900nm bands all show diminished accuracy at low oxygen saturations.

BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION

According to exemplary embodiments of the invention, more accurateestimates of low arterial oxygen saturation using pulse oximetry areachieved by optimizing a wavelength spectrum of first and second lightsources so that the saturation estimates at low saturation values areimproved while the saturation estimates at high saturation values areminimally adversely affected as compared to using conventional first andsecond wavelength spectrums. It has been discovered that calculations atlow saturation can be significantly improved if the anticipated orpredicted rates of absorption and scattering of the first wavelengthspectrum is brought closer to, optimally equal to, the anticipated orpredicted rates of absorption and scattering of the second wavelengthspectrum than otherwise exists when conventional wavelength spectrumpairs are chosen, such as when conventionally using a first wavelengthcentered near 660 nm and a second wavelength centered anywhere in therange of 880 nm-940 nm.

The present techniques solve a long felt need for a pulse oximetersensor and system which provides more accurate estimates of arterialoxygen saturation at low oxygen saturations, i.e. saturations equal toor less than 80%, 75%, 70%, 65%, or 60%, than has heretofore existed inthe prior art. The sensor and system is particularly useful forestimating arterial saturation of a living fetus during labor where thesaturation range of principal importance and interest is generallybetween 15% and 65%, and is particularly useful for estimating arterialsaturation of living cardiac patients who experience significantshunting of venous blood into their arteries in their hearts and hencewhose saturation range of principle importance and interest is roughlybetween 50% and 80%. By contrast, a typical healthy human has asaturation greater than 90%. The invention has utility whenever thesaturation range of interest of a living subject, either human oranimal, is low.

In addition to providing better estimates of arterial oxygen saturationat low saturations, the sensor, monitor, and system disclosed herein mayprovide better and more accurate oxygen saturation estimates whenperturbation induced artifacts exist and are associated with the subjectbeing monitored.

When the rates of absorption and scattering by the tissue being probedby the first and second wavelength spectrums are brought closer togetherfor the saturation values of particular interest, improvedcorrespondence and matching of the tissue actually being probed by thefirst and second wavelengths is achieved, thus drastically reducingerrors introduced due to perturbation induced artifacts. For example,when light of one wavelength is absorbed at a rate significantly higherthan that of the other wavelength, the light of the other wavelengthpenetrates significantly further into the tissue. When the tissue beingprobed is particularly in-homogenous, this difference in penetrationscan have a significant adverse impact on the accuracy of the arterialoxygen saturation estimate.

Perturbation induced artifacts include, but are not limited to, anyartifact that has a measurable impact on the relative optical propertiesof the medium being probed. Perturbation induced artifacts include butare not limited to the following:

-   -   (1) variations in the tissue composition being probed by the        sensor from subject to subject, i.e., variations in the relative        amounts of fat, bone, brain, skin, muscle, arteries, veins,        etc.;    -   (2) variations in the hemoglobin concentration in the tissue        being probed, for example caused by vasal dilations or vasal        constrictions, and any other physical cause which affects blood        perfusion in the tissue being probed; and    -   (3) variations in the amount of force applied between the sensor        and the tissue being probed, thus affecting the amount of blood        present in the nearby tissue.

In one embodiment, there is provided a fetal pulse oximeter sensor witha light source optimized for the fetal oxygen saturation range and formaximizing the immunity to perturbation induced artifact. A far red andan infrared light source may be used, with the far red light sourcehaving a mean wavelength between 700-790 nm. The infrared light sourcecan have a mean wavelength as in prior art devices used on patients withhigh saturation, i.e., between 800-1000 nm. As used herein, “highsaturation” shall mean an arterial oxygen saturation greater than 70%,preferably greater than 75%, alternatively greater than 80%, optionallygreater than 90%.

The fetal sensor may be optimized by arranging the spacing between thelocation the emitted light enters the tissue and the location thedetected light exits the tissue to minimize the sensitivity toperturbation induced artifact.

According to one embodiment, electrooptic transducers (e.g., LEDs andphotodetectors) are located adjacent to the tissue where the lightenters and exits the tissue. According to an alternate embodiment, theoptoelectric transducers are located remote from the tissue, for examplein the oximeter monitor, and optical fibers interconnect the transducersand the tissue with the tissue being illuminated from an end of a fiber,and light scattered by the tissue being collected by an end of a fiber.Multiple fibers or fiber bundles are preferred.

The typical oxygen saturation value for a fetus is in the range of5-65%, commonly 15-65%, compared to the 90% and above for a typicalpatient with normal (high) saturation. In addition, a fetal sensor issubject to increased perturbation induced artifact. Another uniquefactor in fetal oximetry is that the sensor is typically insertedthrough the vagina and the precise location where it lands is not knownin advance.

All of these features one unique to fetal oximetry or oximetry for lowsaturation patients and provide a sensor which optimizes the immunity toperturbation induced artifacts. This optimization is done with atrade-off on the sensitivity to changes in saturation value. Thistrade-off results in a more reliable calculation that is not obvious tothose who practice the prior art methods which attempt to maximize thesensitivity to changes in the saturation value. The improvement inperformance that results from these optimizations are applicable to bothreflectance and transmission pulse oximetry. An example of a fetaltransmission pulse oximetry configuration usable with the presentinvention is described in U.S. patent application Ser. No. 07/752,168,assigned to the assignee of the present invention, the disclosure ofwhich is incorporated herein by reference. An example of a non-fetaltransmission pulse oximetry configuration usable with the presentinvention is described in U.S. Pat. No. 4,830,014, assigned to theassignee of the present invention, the disclosure of which isincorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart of the absorption characteristics of oxyhemoglobin(HbO₂) and reduced hemoglobin (Hb) versus wavelength showing prior artnear red and infrared LED wavelengths;

FIG. 2 is a graph of red/IR modulation ratio versus oxygen saturation;

FIG. 3 is a diagram illustrating light penetration through differentlayers of tissue at different distances;

FIG. 4A is a chart of the variation in extinction and scatteringcoefficients over a range of wavelengths for different saturationvalues;

FIG. 4B is a table of the values of FIG. 4A;

FIG. 5 is a diagram illustrating the placing of a sensor on a fetus;

FIG. 6 is a graph illustrating the spectrum of an LED according to anexemplary embodiment of the present invention;

FIGS. 7-18 are graphs showing experimental modeling of the modulationratio and saturation error as a function of saturation for different redand infrared wavelength combinations;

FIGS. 19-23 are charts illustrating saturation and the error due toapplied force for different combinations of emitter wavelength andemitter-detector spacing from experiments done on sheep;

FIGS. 24 and 25 are diagrams illustrating the construction of a sensoraccording to an exemplary embodiment of the present invention;

FIGS. 26A-B are diagrams of a single package, dual emitter package usedin an exemplary embodiment of the present invention; and

FIG. 27 is a block diagram of a pulse oximeter according to an exemplaryembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An understanding of the design of the fetal sensor disclosed hereinbenefits from an understanding of the environment in which the sensorwill operate. FIG. 3 illustrates the layers of tissue in a typical fetuslocation where a sensor may be applied. Typically, there would be afirst layer of skin 12, perhaps followed by a layer of fat 14, a layerof muscle 16, and a layer of bone 18. This is a simplified view forillustration purposes only. The contours and layers can vary atdifferent locations. For instance, bone would be closer to the surfaceon the forehead, as opposed to closer muscle on the neck. Suchvariations in sites can produce the first type of perturbation artifactmentioned in the summary—artifact due to variations in tissuecomposition.

The general paths of light from an emitter 20 to a photodetector 22 areillustrated by arrows 24 and 26. Arrow 24 shows light which passesalmost directly from emitter 20 to detector 22, basically shunted fromone to the other, passing through very little blood perfused tissue.Arrow 26, on the other hand, illustrates the deeper penetration ofanother path of the light. The depth of penetration is affected by thewavelength of the light and the saturation. At low saturation, infraredlight penetrates deeper than near red, for instance. The deeperpenetration can result in an undesirable variation between the infraredand red signals, since the IR signal will pass through more differentlayers.

Also illustrated in FIG. 3 is the effect of using an emitter 28 which isspaced on the tissue at a greater distance from a detector 30 than thefirst pair 20, 22 described. As can be seen, this greater separationresults in the penetration of a larger amount of tissue, as indicated byarrows 32 and 34. Thus, the greater spacing increases the depth ofpenetration, although it will reduce the intensity of the signalreceived at the detector due to more attenuation from more of the lightbeing absorbed in the tissue and the greater light propagation distancesinvolved.

The second type of perturbation mentioned in the summary is variationsin the concentration of blood in the tissue from patient to patient orover time. A lower concentration results in less absorption, increasingthe penetration depth. The inventors estimate that the mean penetrationdepth of photons in a medium is related to the product of the absorptionand scattering coefficients, and this estimate is consistent with thefindings of Weiss et al., “Statistics of Penetration Depth of PhotonsRe-emitted from Irradiated Tissue”, Journal of Modern Optics, 1989, Vol.36, No. 3, 349-359, 354, the disclosure of which is incorporated hereinby reference.

Absorption of light in tissue in the visible and near infrared region ofthe electromagnetic spectrum is dominated by the absorptioncharacteristics of hemoglobin. Absorption coefficients of hemoglobin canbe found in the literature, for example Zijlstra et al., “Absorptionspectra of human fetal and adult oxyhemoglobin, de-oxyhemoglobin,carboxyhemoglobin and methemoglobin”, Clinical Chemistry, 37/9,1633-1638, 1991 (incorporated herein by reference). Measured scatteringcoefficients of tissue are influenced by the methodology of measurementand the model used to fit the data, although there is general agreementin the relative sensitivity to wavelength regardless of method. Tissuescattering coefficients used by the inventors are based on diffusiontheory, and are taken from Schmitt, “Simple photon diffusion analysis ofthe effects of multiple scattering on pulse oximetry”, IEEE Transactionson Biomedical Engineering, Vol. 38, No. 12, December 1991, thedisclosure of which is incorporated herein by reference.

FIG. 4A is a graph showing the product of the absorption and scatteringcoefficients for 0%, 40%, 85% and 100% saturations for wavelengthsbetween 600 nm and 1,000 nm. For 85-100% tissue oxygen saturation, goodbalance or correlation exists between the product of the absorption andscattering coefficients of conventionally chosen wavelength pairs (i.e.,660 nm and 892 nm), as illustrated by points A and B on curve 101.

For low tissue oxygen saturation, points C and D on curve 102graphically indicate that there is a very significant mismatch betweenthe product of the absorption and scattering coefficients of the 660 nmnear red and 892 nm infrared light, with the near red light being morestrongly absorbed and scattered. This very significant absorption andscattering mismatch results in very different tissue being probed by thenear red and infrared light which significantly degrades the accuracy ofthe arterial oxygen saturation calculation. In addition, when a largerange of low arterial oxygen saturations need to be accuratelycalculated, as when monitoring a fetus during labor where the range ofarterial oxygen saturations can extend between 15% and 65%, it isevident from FIG. 4A that not only does a significant mismatch betweenthe rates of absorption and scattering of the near red and infraredlight exist, but that the amount of mismatch will vary significantly asarterial oxygen saturation varies, thus causing a differentialinaccuracy of oxygen saturation estimates which varies with the arterialsaturation.

On the other hand, points D and E on curve 102 in FIG. 4A illustrateadvantages of a preferred embodiment of the invention of choosing firstand second wavelengths, i.e., 732 nm and 892 nm, which have absorptionand scattering characteristics which are more closely balanced ascompared to the prior art pairing of 660 nm and 892 nm for 40% tissueoxygen saturation. As can be appreciated, since the 732 nm extinctionand scattering coefficients more nearly match the 892 nm extinction andscattering coefficients, improved overlap of the tissue being probed bythe two wavelengths of light result. In addition, 732 nm results in asmaller variation of the extinction and scattering coefficients as afunction of oxygen saturation as compared to 660 nm, thus resulting inbetter and more accurate oxygen saturation estimates over a wider rangeof saturations. The tissue oxygen saturation values shown in FIG. 4A areclosely correlated to arterial oxygen saturation values. In general, agiven value of tissue oxygen saturation corresponds to a higher value ofarterial oxygen saturation. For example, the inventors estimate that 85%tissue oxygen saturation corresponds to roughly 100% arterial oxygensaturation.

One embodiment optimizes the wavelengths used for a sensor to estimatefetal arterial oxygen saturation during labor where the saturation ofinterest is below 70%, a typical range of interest being between 15% and65%. Attempting to match or balance the rates of absorption andscattering of the two wavelengths in a fetal sensor is particularlyuseful since the amount of perturbation induced artifacts is so severein number and magnitude. For example, for a surface reflection sensor,it is difficult to know a priori where on the fetus the sensor will belocated. For example, sometimes it will be on the head, other times thecheek. Hence, the tissue composition varies from application toapplication. In addition, the force by which the sensor is applied willvary during labor thus introducing still additional perturbation inducedartifacts.

Another embodiment is to use the sensor for cardiac patients whose rangeof saturation, where accuracy in calculations is important, is from 50%to 80%.

FIG. 5 illustrates the placement of a sensor 410 on a fetus 412. Thesensor is connected by a cable 414 to an external pulse oximetermonitor. As can be seen, sensor 410 is wedged between a uterine wall 416and the fetus 412. In this instance, the sensor is on the side of thefetus' forehead. This wedging of the sensor applies a force to the skinimmediately below the sensor, which reduces the amount of blood in thelocal tissue. This reduces the amount of blood the light signal willpass through, thus increasing the difficulty of obtaining an accurateblood oxygenation reading.

In choosing an optimum LED wavelength, it should be kept in mind thatLEDs have a spectral width, and are not a single narrowband wavelengthdevice like a laser. FIG. 6 illustrates the spectral spread of onewavelength for a sensor, showing the far red wavelength at 735 nm asbeing the peak wavelength. However, arrow 510 indicates a distributionof wavelengths which can be approximately 25 nm wide at which theintensity level is approximately 50% of that of the peak wavelength. Inaddition, when manufacturing LEDs, it is difficult to tightly controlthe mean wavelength. Thus, a purchaser specifying a particularwavelength, such as a 735 nm wavelength in an embodiment of the presentinvention, will expect to receive LEDs whose actual mean wavelength canvary by 10, 20 or more nanometers from the specified value. A narrowrange is typically achieved by testing and sorting.

FIG. 27 is a block diagram of one embodiment of a pulse oximeterimplementing the present invention. Light from light source 210 passesinto patient tissue 212, and is scattered and detected by photodetector214. A sensor 200 containing the light source and photodetector may alsocontain an encoder 216 which provides signals indicative of thewavelength of light source 210 to allow the oximeter to selectappropriate calibration coefficients for calculating oxygen saturation.Encoder 216 may, for instance, be a resistor.

Sensor 200 is connected to a pulse oximeter 220. The oximeter includes amicroprocessor 222 connected to an internal bus 224. Also connected tothe bus is a RAM memory 226 and a display 228. A time processing unit(TPU) 230 provides timing control signals to light drive circuitry 232which controls when light source 210 is illuminated, and if multiplelight sources are used, the multiplexed timing for the different lightsources. TPU 230 also controls the gating-in of signals fromphotodetector 214 through an amplifier 233 and a switching circuit 234.These signals are sampled at the proper time, depending upon which ofmultiple light sources is illuminated, if multiple light sources areused. The received signal is passed through an amplifier 236, a low passfilter 238, and an analog-to-digital converter 240. The digital data isthen stored in a queued serial module (QSM) 242, for later downloadingto RAM 26 as QSM 242 fills up. In one embodiment, there may be multipleparallel paths of separate amplifier filter and A/D converters formultiple light wavelengths or spectrums received.

A detector and decoder module 242 determines the wavelength of the lightsource from encoder 216. One embodiment of circuitry for accomplishingthis is shown in commonly assigned U.S. Pat. No. 4,770,179, thedisclosure of which is hereby incorporated by reference.

Based on the value of the received signals corresponding to the lightreceived by photodetector 214, microprocessor 222 will calculate theoxygen saturation using well-known algorithms. These algorithms requirecoefficients, which may be empirically determined, corresponding to, forexample, the wavelengths of light used. These are stored in a ROM 246.The particular set of coefficients chosen for any pair of wavelengthspectrums is determined by the value indicated by encoder 216corresponding to a particular light source in a particular sensor 200.In one embodiment, multiple resistor values may be assigned to selectdifferent sets of coefficients. In another embodiment, the sameresistors are used to select from among the coefficients appropriate foran infrared source paired with either a near red source or far redsource. The selection between whether the near red or far red set willbe chosen can be selected with a control input from control inputs 254.Control inputs 254 may be, for instance, a switch on the pulse oximeter,a keyboard, or a port providing instructions from a remote hostcomputer.

Both modeling and prototypes may be used to achieve the optimized sensorset forth herein. Several theoretical models exist for describing thescattering of light within tissue. The models used by the inventorsassume isotropic scattering within a homogeneous tissue bed. Even thoughthis is a simplification of the true nature of light scattering intissue (tissue is inhomogeneous and light is scattered primarily in theforward direction), these models are useful for predicting behaviors ofpulse oximetry, and the sensitivity to many design parameters.

Utilizing such a model, different choices of LED wavelengths wereexplored. Tissue characteristics were numerically defined and the basis(calibration) correlation between SaO₂ and modulation ratio wascalculated for each wavelength pair considered. Change in physiologicalcondition was simulated by revising one or more of the numericallydefined physical parameters. SpO₂ was recalculated from the resultingmodulation ratio, and the saturation region where errors were minimizedwas noted. For arterial saturations above 80% the conventionalwavelength choice of 660 nm paired with 890 nm results in optimumperformance, while for arterial saturations below 70%, 735 nm bandemitters paired with 890 nm gives improved stability.

FIGS. 7 through 18 show the predicted errors due to changing the tissueblood volume to one fourth the basis value for a variety of red and IRLED wavelength pairs. The A figures (such as 7A) show the modulationratio vs. SaO₂. The B figures (7B) show the saturation error vs. SaO₂.This perturbation simulates the effects of blood volume variationswithin the patient population, anemia, ischemia, or localizedexsanguination of blood in the tissue.

Sensitivity of the calibration to a change in tissue blood concentrationis shown for several pairings of red and IR wavelengths. In each case,the LED has no secondary emission, and the perturbation is in going froma nominal 2% blood concentration in the tissue to 0.5%. Figure Table IRLED red LED 805 nm 890 nm 940 nm 660 nm 7 8 9 700 nm 10 730 nm 11 12 13760 nm 14 15 16 790 nm 17 18

FIGS. 7-9 show the type of performance found in conventional pulseoximeters. FIGS. 10-18 show shifting of the optimum performance regionfrom saturations above 80% to lower saturations when the red LEDwavelength is chosen in the 700-790 nm region of the spectrum. Lightscattering is minimally affected by changes in oxygenation, but lightabsorption is significantly affected as reduced hemoglobin in the tissuechanges to oxyhemoglobin or vice-versa. Pulse oximetry's optimumperformance region occurs when there is a balance of the two channels'scattering and absorption properties within blood perfused tissue.Balance occurs when there is a good overlap of the volumes of tissueprobed by the two channels, requiring that the penetration depth oflight at the two wavelengths be matched. At the higher saturations, thisoptimum balance occurs with the pairing of wavelengths with a redemitter in the 660 nm band, while at the lower saturations the balanceimproves with the use of a red emitter in the 730 nm band. The variationof the IR LED from 805 to 940 nm does not produce a significantdifference in performance.

When using an LED pair near 730 nm and 890 nm for pulse oximetry, thesensitivity of modulation ratio to changes in oxygen saturation (i.e.,the slope of the curve in, for example, FIG. 1) is reduced relative tothe use of 660 nm and 890 nm, but the measurement becomes more robust tochanges in the tissue characteristics other than oxygen saturation.Noise in the measurement of modulation ratio due to factors such asinstrument electronics noise, digitization, or ambient lightinterference, become more important but can generally be accounted forwith good instrument design and appropriate signal processing. The biasand deviations due to tissue optical properties, however, become lesssignificant with the proper choice of emitter wavelengths when they arechosen based on the saturation region of primary interest.

Empirical tests on sheep were conducted using prototype sensors. Theempirical observations support the use of 735 nm band red LEDs in thedesign of a pulse oximeter that is more robust to perturbation inducedartifacts at the lower saturation region. Reflectance pulse oximetrysensors were fabricated using conventional 660-890 nm LED pairs, andwith 735-890 nm pairs.

FIGS. 19-23 show that measurements were taken at a range of oxygensaturation values indicated along the X axis from approximately 100%oxygen saturation to less than 10%. The plots show the calculatedsaturation (SpO2) for each actual saturation (SaO2) value. The actualsaturation value is determined by simultaneously drawing blood samplesfrom an arterial catheter placed in the left femoral artery. SaO₂ ismeasured on a laboratory co-oximeter (Instrument Labs IL 282 orRadiometer OSM-3). This is the value used on the X axis in thesefigures.

As can be seen, the diagonal line in FIGS. 19, 20, and 22 indicates thedesired result where the calculated value is equal to the actual valueas measured with the catheter. The tests illustrated in FIGS. 19, 20,and 22 were done with a nominal force of approximately 50 grams appliedto the sensor holding it against the skin.

Using the 660 nm sensor with center-to-center emitter/detector spacingof 14 mm at the tissue, FIG. 19 shows that sensor calibration is verysensitive to the type of tissue probed. The calibration on the head andneck are very different.

Using the 735 nm sensor with a 5.8 mm center-to-center emitter/detectorspacing at the tissue, the bias between the head and neck is greatlyreduced as illustrated by FIG. 20. There is, however, still substantialsensitivity to surface exsanguination. This is apparent in FIG. 21 whichillustrates the effect of a perturbation induced artifact (sensorapplied force).

FIG. 22 shows the location insensitivity of a 735 nm sensor with a 14 mmcenter-to-center emitter/detector spacing. FIG. 23 shows that thissensor is also insensitive to force applied to the sensor (perturbationinduced artifact).

It was experimentally confirmed that increasing the emitter/detectorcenter-to-center spacing from 5.8 mm for 735 nm/890 nm LED wavelengthsdecreased the sensitivity to perturbation induced artifacts, with goodperformance being achieved by an emitter/detector separation equal to orgreater than 10 mm.

Both the modeling and the actual experiments illustrate an improvementin reliability of a saturation measurement achieved by optimizing thered wavelength to be within 700-790 nm range. In addition, reduction ofthe saturation error reading in the presence of force artifact isachieved by increasing the spacing of the emitters from the detector.

The force applied to the sensor causes exsanguination of the surfacetissue, further magnifying the remaining disparities due to theinhomogeneity of the tissue, or causing shunting of light between theemitter and detector, thus causing errors in the saturation calculation.These are compensated for by wider emitter/detector spacing, whichresults in the light from the red and infrared LEDs penetrating deeperinto the tissue, thus increasing the likelihood of their going through,on the average, the same combination of tissue structures, asillustrated in FIG. 3.

FIG. 24 is a top view of a sensor according to one embodiment of thepresent invention. The sensor face 110 supports a far red LED 112 and aninfrared LED 114. These are spaced by a distance of 14 mmcenter-to-center from a detector 116. Preferably, the centers of the farred and infrared LEDs are no more than 0.5 mm apart. The sensor face isconnected by a cable 118 to a connector 120 for connection to the pulseoximeter monitor. FIG. 25 shows a side view of the sensor of FIG. 24,illustrating the fulcrum portion 122 of the sensor and sensor back 132.When placed in utero, the uterus will apply a force to the sensor back132 and deform the fulcrum 122. As can be seen, this technique resultsin a force being applied to the sensor resulting in good sensor-fetuscontact but possibly resulting in local exsanguination of the tissue. Itshould be noted that any sensor embodiment will have possible localexsanguination.

The modeling and empirical tests show that the nature of the correlationbetween modulation ratio and saturation in pulse oximetry is related totissue optical properties, and that the sensitivity to varyingperturbation induced artifacts can be affected by choice of emitterwavelengths. For high oxygen saturations, the choice of 660 nm and 890nm band emitters is well suited for stable pulse oximetry calculations,while 700-790 nm and 890 nm band emitters perform better at lowsaturations. Other wavelength combinations may be chosen from elsewherein the visible and near infrared portion of the spectrum by following ananalysis similar to the one described here. Currently, however, overallinstrument design considerations (e.g., electronic signal-to-noise andpotential shunting of light with narrowly spaced components in areflectance probe) favor the use of the wavelengths discussed. By usingthe analysis described, other improvements to pulse oximetry arepossible. FIGS. 19-23 illustrate the results of these tests for severalprototype sensors.

FIGS. 26A and 26B are front and side views of a single packagecontaining emitters 112 and 114 of FIGS. 24 and 25. Both emitters areencapsulated in a single semiconductor package, to make the package morecompact to provide the miniaturization which is advantageous for a fetalsensor application. In the embodiment of FIG. 26A, emitter die 112 ismounted via a conductive epoxy 130 to a substrate 132. Substrate 132takes the form of a metal plating, an exterior portion 134 of whichforms the outside lead to the package. Emitter 114 is mounted on top ofmetal substrate 136, an exterior 138 of which forms the second lead.

The electrical connection to emitter 114 is provided through lead 138 onone side up through the conductive epoxy, and through the other side viaa wire bond 140, which connects to the other lead 134. Similarly, lead134 connects through conductive epoxy 130 to the second emitter 112,with the other side of emitter 112 connected via a wire bond 142 to lead138. Accordingly, as can be seen, applying a voltage with a firstpolarity to the two leads 134 and 138 will turn on one of the emitters,and turn off the other, while reversing the polarity will reverse whichemitter is turned on and which emitter is turned off. Both of theemitters and their corresponding substrates are encapsulated in apackage 144 which may, for instance, be plastic.

FIG. 26B is a side view showing the encapsulated package 144 from theside, and illustrating the emitting light 146 from emitters 112, 114.The structure of FIGS. 26A-26B is compact and usable for a fetalapplication. The distance between the centers of the two emitter dies112 and 114 may be less than 2 mm. This way the package's wiring allowsthe package to have two leads, as opposed to four leads which would berequired by using two separate emitter packages.

As an alternative to using a far red and an infrared LED, other methodsfor producing selected light spectrums of two different wavelengths canbe used. For example, lasers could be used rather than LEDs.Alternately, a white light or other light source could be used, with thewavelength being optimized at the detector. This could be done by usingappropriate filters in front of either the light source or the detector,or by using a wavelength sensitive detector. If filters are used, theycould be placed in front of alternate detectors or emitters, or filterscould be alternately activated in front of a single emitter or detector.

A pulse oximeter for use over a broad saturation range can utilizemultiple wavelength pairs (e.g., both 660 nm and 730 nm band emitterscoupled with a 900 nm emitter), with the appropriate emitter pair chosenfor use in the calculation of SpO₂ based on the estimated value of theoxygen saturation.

Such a pulse oximeter could be implemented with two or more red LEDs, oralternately could be implemented with a single light source and multiplefilters, or multiple wavelength sensitive detectors. Different redwavelength spectrums could be utilized, based on the saturation of thepatient.

As will be understood by those with skill in the art, the presentinvention can be embodied in other specific forms without departing fromthe essential characteristics thereof The wavelength could be variedwhile still optimizing in accordance with the present invention. Also,light pipes, light fibers, multiple filters, or multiple detectors couldbe used in accordance with the concepts of the present invention.Different sensors than the fulcrum structure as set forth in FIG. 25could be used, such as a bladder structure for inflating and holding thesensor against the fetus. Accordingly, reference should be made to theappended claims for defining the scope of the invention.

1. A method, comprising: emitting infrared light from a first lightsource into tissue, the infrared light including an infrared wavelengthspectrum useful for measuring oxygen saturation in a patient with highsaturation; emitting far red light from a second light source into thetissue, the far red light including a wavelength between 700 and 790nanometers; and detecting the infrared light and far red light from thefirst and second light sources with a detector after the light has beenscattered by the tissue.
 2. The method of claim 1, comprisingdetermining a blood oxygen saturation value based on an intensitymeasurement of the detected light.
 3. The method of claim 1, comprisingemitting red light from a third light source, wherein the red light hasa mean wavelength less than 700 nanometers.
 4. The method of claim 3,comprising activating one or all of the first second and third lightsources.
 5. A pulse oximeter sensor, comprising: a first light sourceconfigured to emit infrared light into tissue, the infrared lightincluding an infrared light spectrum with a range useful for measuringoxygen saturation in a patient with high saturation; a second lightsource configured to emit red light into the tissue, the red lightincluding a red light spectrum having a mean wavelength between 700 and790 nanometers; a detector configured to detect the red and infraredlight after the red and infrared light are scattered by the tissue; anda limiting component configured to limit light signals received at thedetector from the light source to three or less spectra.
 6. The pulseoximeter sensor of claim 5, comprising a housing in which the first andsecond light sources and the detector are mounted.
 7. The pulse oximetersensor of claim 6, wherein the first and second light sources compriseinfrared and red light emitters spaced at least 10 millimeters apart. 8.The pulse oximeter sensor of claim 6, wherein the first and second lightsources comprise infrared and red light emitters spaced at least 14millimeters apart.
 9. The pulse oximeter sensor of claim 5, wherein thelimiting component comprises a filter disposed between the first andsecond light sources and the detector.
 10. The pulse oximeter sensor ofclaim 5, wherein the limiting component comprises a wavelength sensitivedetector that detects a limited spectrum of light.
 11. The pulseoximeter sensor of claim 5, wherein the limiting component comprises awavelength limited light emitter that emits a limited light spectrum.12. The pulse oximeter sensor of claim 5, wherein the light sourcecomprises: a first light emitting diode configured to emit light in theinfrared light spectrum with the range useful for measuring oxygensaturation in the patient with high saturation; and a second lightemitting diode configured to emit light in the red light spectrum havinga mean wavelength less than 700 nanometers.
 13. A pulse oximeter sensor,comprising: a light source configured to emit light; a detectorconfigured to detect the light; a limiting component configured to limitthe detected light to three or less spectra, wherein the three or lessspectra include a first spectrum including 735 nanometers at anintensity of at least 50% of the intensity of any other wavelengths inthe first spectrum.
 14. The pulse oximeter sensor of claim 13, whereinthe three or less spectra include a second spectrum having a meanwavelength of from 805 to 940 nanometers.
 15. The pulse oximeter sensorof claim 14, wherein the three or less spectra include a third spectrumhaving a mean wavelength of about or near 660 nanometers.
 16. A method,comprising: providing a light source configured to emit light intotissue, the light including an infrared light spectrum with a rangeuseful for measuring oxygen saturation in a patient with highsaturation, and the light including a red light spectrum having a meanwavelength between 700 and 790 nanometers; providing a detectorconfigured to detect the light after the light is scattered by thetissue; and providing a limiting component configured to cooperate withone or both of the detector and the light source, the limiting componentconfigured to limit light signals received at the detector from thelight source to three or less spectra.
 17. The method of claim 16,comprising mounting the light source and detector in a housing.
 18. Themethod of claim 16, comprising providing red and infrared light emittersspaced at least 10 millimeters apart as the light source.
 19. The methodof claim 16, comprising disposing a filter between the light source anddetector.
 20. The method of claim 16, comprising: providing a firstlight emitting diode configured to provide the infrared light spectrumwith the range useful for measuring oxygen saturation in the patientwith high saturation; and providing a second light emitting diodeconfigured to provide a red light spectrum having a mean wavelength lessthan 700 nanometers.
 21. A pulse oximeter system, comprising: a pulseoximeter configured to read signal data and derive patient data from thesignal data; and a sensor, comprising: a light source configured to emitlight into tissue, the light including an infrared light spectrum with arange useful for measuring oxygen saturation in a patient with highsaturation, and the light including a red light spectrum having a meanwavelength between 700 and 790 nanometers; a detector configured todetect the light after the light is scattered by the tissue and totransmit the signal data to the pulse oximeter; and a limiting componentconfigured to limit light signals received at the detector from thelight source to three or less spectra and to transmit the signal data tothe pulse oximeter.